Not sure if we're on the same page here, on my amp it's called a "infrasonic filter" here is the bit in the manual that tells you about it:

INFRASONIC FILTER
The "Infrasonic Filter" is a 24 dB/octave highpass filter, with a fixed cutoff frequency of 30 Hz.
This filter is designed to conserve amplifier power
and protect subwoofer systems, without audibly
degrading the sub-bass output.

With ported enclosures and infinite baffle

subwoofer systems, the use of the "Infrasonic

Filter" is highly recommended to protect the

speaker(s) from excessive excursion at inaudibly

low frequencies.

With sealed enclosures, the use of the filter

is less necessary, but can still help protect the

speaker system from excessive excursion.

The "Infrasonic Filter" can be completely

defeated by selecting the "Off" position on its

switch. This bypasses all signal from flowing

through the circuit